1. Field of the Invention
The present invention relates generally to high precision linear positioning apparatus and more particularly to such apparatus as utilized for the purpose of positioning magnetic record/playback heads used to transfer data to and from flexible magnetic disks.
2. Description of the Prior Art
Disk drive systems that record data on and read data from flexible magnetic disks, or floppy disks as they are commonly known, are composed of several elements. The floppy disk itself is a thin mylar or polyester disk that is coated with a magnetic material and is enclosed in a protective envelope having access holes for spinning the disk and for accessing the magnetic surface. Disk spinning is accomplished by a motor driven spindle, while access to the magnetically encoded data is provided by magnetic record/playback head attached to a carriage that is positioned radially along a guide track by a drive mechanism. Digital data is recorded on the floppy disk in concentric recording tracks by positioning the record/playback head in contact with the spinning magnetic surface at the appropriate radial location and by electromagnetically exciting the record windings of the head thereby aligning the magnetic particles in the underlying surface coating in patterns corresponding to the digital data. Playback of the record recorded data is accomplished by sensing the electromagnetic response of the playback windings of the head.
A trend toward higher data storage density has necessitated increasingly tighter tolerances on head positioning accuracy to allow closer intertrack spacing and, hence, more data storage per floppy disk. Precise head positioning throughout the service life of a disk drive, as well as accurate head positioning, referenced to a standard to allow floppy disk interchangeability among all disk drives, is desired.
Many disk drives utilize two magnetic record/playback heads and magnetic coatings on both surfaces of the floppy disk to increase data storage density. To insure the accurate transfer of data to and from the floppy disk, the contact force between the heads and the magnetic surface must be adjusted to and maintained at an optimum level. If the contact force is too high, excessive abrasion of the disk surface and consequently reduced service life will result, whereas a contact force that is too low may allow head bounce, and loss of data due to variations in disk thickness. Provisions must also be made to separate the upper and lower record/playback heads to allow for floppy disk loading and unloading.
What is needed therefore, is a precision record/playback head positioning apparatus that will accurately and consistently position the head, or heads, throughout the service life of the disk drive. What is additionally needed is a mechanism which allows an optimum contact force to exist between the record/playback heads and the magnetic surface of the floppy disk.
Another aspect of the present invention relates to devices utilizing screw threads for positioning or clamping purposes in which clearances between mating screw threads is a concern. A threaded positioning device typically employs an externally threaded shaft, or lead screw, and an internally threaded follower, or nut, which is translated along the axis of the shaft in response to shaft rotation. A clearance between mating screw threads is necessary to allow operation of the device without binding. However, this clearance also tends to decrease the positioning accuracy of the device.
Threaded fasteners used for clamping also suffer from thread clearance problems. In a situation where a threaded bolt is loaded in tension by a nut, a clamping force is generated through compressive loading of those screw thread flanks of the bolt that face toward the bolt head and are in contact with screw thread flanks of the nut that face away from the bolt head. Only one set of the two sets of facing flanks is normally in contact due to the clearances between the screw threads. Locking of the nut in place is accomplished by a friction force component of the contact force existing between contacting flanks, but due to the pitch of the screw, this contact force also has a loosening force component which tends to loosen the nut. Threaded fasteners used in vibratory situations therefore require additional means to insure locking.
Consequently, what is needed is a method for providing clearance between the screw threads of two mating devices so as to allow rotation of one device with respect to the other while at the same time maintaining accurate axial positioning. What is also needed is a method for locking mating screw threads by utilizing both sets of facing screw thread flanks to provide a clamping force free from any loosening force components.
Guidance and drive mechanisms are known in the prior art. For example, U.S. Pat. No. 3,946,439 and the "IBM Technical Disclosure Bulletin", Volume 18, Number 7, December 1975, pp. 2244-2245, both disclose record/playback head positioning apparatus that use parallel rods for lateral guidance, and tension band drives for radial positioning. A carriage, upon which a head is mounted, is guided laterally by one fixed rod while another fixed rod prevents rotation of the carriage. Such guidance mechanisms require clearances between the carriage and the rods to allow radial movement of the carriage without binding. However, these clearances reduce the attainable positioning accuracy of the apparatus. Furthermore, increases in clearances due to wear will even further reduce the positioning accuracy.
The tension band drive mechanism consists of a continuous loop, or band, that is attached to carriage and loops around a motor driven pulley and a tensioning pulley. The carriage is pulled by the tension band, in one direction or the other, in response to motor rotation. Inaccuracies due to hysteresis in the drive mechanism when reversing directions limits the precision of this approach.
An alternate guidance mechanism including a grooved track and follower is shown in U.S. Pat. No. 3,947,886. As disclosed, a carriage is spring loaded against a U-shaped track to provide lateral positioning. The positioning accuracy of this approach is susceptible to any accumulation of dirt in the track area and also to wear of the track or the track following pins.
Other prior art head positioning apparatus utilize motor driven lead screws to move and position carriage mounted record/playback heads. See, for example, U.S. Pat. Nos. 3,678,481 and 4,030,137. The carriage in each of the above mentioned patents translates along the axis of the lead screw in response to rotation of the lead screw. A fixed rod, parallel to the axis of the lead screw restricts carriage rotation. Running clearance and wear between the lead screw and its follower are again limiting factors in the positioning accuracy of this type of head positioning apparatus.
A linear bearing of a type having reduced wear related problems and loss of accuracy is disclosed in U.S. Pat. No. 2,952,145. While this linear bearing reduces loss of positioning accuracy due to wear of the guidance mechanism, no provisions are disclosed for reducing the running clearances to those required of a high precision positioning apparatus.
Spring loaded devices that provide for contact between a record/playback head and a floppy disk are disclosed in U.S. Pat. Nos. 3,678,481 and 3,946,439. The former describes a carriage with a single head and spring loaded pressure pad, while the latter shows a carriage with two heads that are forced together by two non-adjustable springs.
Although mechanisms exist in the prior art which tend to improve the positioning accuracy of threaded positioning devices by biasing a nut follower to eliminate the clearance between one set of facing screw thread flanks of mating threads, typically by using a spring to continuously bias the follower in one axial direction, long term accuracy is poor due to wear of the mating thread flanks.